Process for making asphalt paving compositions

ABSTRACT

PARTICULATE EMISSIONS DURING PRODUCTION OF ASPHALT CONCRETE ARE CONTROLLED BY CONTRACTING WET AGGREGATE WITH AN ASPHALT COMPOSITION AT OR BEFORE INTRODUCTION OF THE AGGREGATE INTO A HEATED MIXING AND DRYING ZONE WHEREIN THE ASPHALT-AGGREGATE MIXTURE IS MOVED THROUGH A RELATIVELY HIGH VELOCITY, HEATED GAS STREAM IN THE MIXER FLOWING PARALLEL TO THE DIRECTION OF MOVEMENT OF THE ASPHALT MIXTURE THROUGH THE MIXING AND DRYING ZONE. THE PROCESS, IN ADDITION TO CONTROL OF PARTICULATE EMISSIONS, RESULTS IN LESS DAMAGE TO THE ASPHALT DUE TO AGING OR HARDENING, AND THE ABILITY TO ACHIEVE A MORE UNIFORM MIXTURE COMPACTION OF THE SAID ASPHALT CONCRETE BECAUSE THE PARALLEL FLOW ENABLES MORE UNIFORM TEMPERATURE CONTROL OF THE ASPHALT AND AGGREGATE MIXTURE ON DISCHARGING FROM THE MIXING AND DRYING ZONE. MEANS FOR AUTOMATING PRODUCTION OF THE PAVING COMPOSITIONS ARE DISCLOSED.

Aug. 27, 1974 H. N. SHEARER 3,832,101

PROCESS FOR MAKING ASPHALT PAVING COMPOSITIONS Filed Nov. 5, 1972 3ShQBtS-ShBit 1 Aug. 27, 1974 H. N. SHEARER PROCESS FOR MAKING ASPHALTPAVING COMPOSITIONS m MZEH 3 m m ommm .r zma out. n o s I I S t e a a owE SV a 205x525 v m ommm o X=Z 20mm zwm ommm oomm Ow in PK M 2 7 9 l "orx M 8 d e l i or g- 27, 1974 H. N. SHEARER 3,832,201

PROCESS FOR MAKING ASPHALT PAVING COMPOSITIONS Filed Nov. 3, 1972 3Sheets-Sheet 5 PASSING NO. 200 IN AGGREGATE 6 -O 4 r O PASSING NO. 200 mMIX FIGo 5 United States Patent US. Cl. 106281 6 Claims ABSTRACT OF THEDISCLOSURE Particulateemissions during production of asphalt concreteare controlled'by contacting Wet aggregate with an asphalt compositionat or before introduction of the aggregate into a heated mixing anddrying zone wherein the asphalt-aggregate mixture is moved through arelatively high velocity, heated gas stream in the mixer flowingparallel to the direction of movement of the asphalt mixture through themixing and drying zone. The process, in addition to control ofparticulate emissions, results in less damage to the asphalt due toaging or hardening, and the ability to achieve a more uniform mixturecompaction of the said asphalt concrete because the parallel flowenables more uniform temperature control of the asphalt and aggregatemixture on discharge from the mixing and drying zone. Means forautomating production of the paving compositions are disclosed.

cRoss REFERENCE TO RELATED APPLICATIONS BACKGROUND THE INVENTION Fieldof the Invention vThis invention relates to animproved process andapparatus for producing asphalt-concrete.

Prior Art Relating to the Disclosure Bituminous pavements of the plantmix type have generally been produced by. drying aggregate in a drumdryer, feeding the dried aggregate into a screening unit capable ofseparating the dried aggregate into fractions held in a series ofstorage bins,.feeding the fractions in proportioned amounts from the binunits, and mixing the proportioned amounts with a predetermined amountof bituminous composition in a mixing unit of the pug mill type. Thedrum dryers used to dry the aggregate are generally heated by a forcedair burner. Particulate emissions from dryers of this type have posed aproblem due to increasingiy'sum em air pollution regulations. Dustcollectors of the baghouse type', costing thousands of dollars perplant, are presently themeans used to control particulate emissions fromdrum dryers used in drying aggregate.

Asphalt paving compositions have heretofore been made in a continuousdrum type mixing plant, as disclosed in US. Pats. ,Nos. 2,626,875 and3,423,222, by feeding aggregate into the mixer, contacting it in themixer with a bituminous composition in the form of an emulsion, andheating the'mixture, during mixing of the aggregate and bituminouscomposition, with a heated gas stream flowing countercurrentto the flowof the aggregate-bituminous composition flow.

. SUMMARY OR THE INVENTION One of theobjects of this invention is toprovide a process and "apparatus for making asphalt paving composi-3,832,201 Patented Aug. 27, 1974 tions wherein cold, wet aggregate iscontacted with a liquefied asphalt composition just prior to orconcurrently with introduction of the aggregate into a relatively highvelocity, heated gas stream flowing parallel to the direction of theaggregate-asphalt mixture through a mixing and drying zone. In theheated atmosphere, the mixture of aggregate and asphalt is mixed whilebeing heated to coat the aggregate particles with asphalt and the coatedparticles cascaded through the heated atmosphere to remove moisturetherefrom without substantial aging or hardening of the asphalt Theasphaltcoated aggregate discharged from the heated atmosphere generallyhas a moisture content of less than 0.3 by weight.

Further objects of this invention are: (1) to provide a process forcoating an unheated, wet aggregate with a liquefied asphalt; (2) toprovide a process of coating aggregate with liquefied asphaltcomposition wherein the amount of air pollutants issuing from the mixingdrum are minimized; (3) to provide a process of coating aggregate withliquefied asphalt wherein there is less adverse aging or hardening ofthe asphalt; (4) to provide an improved apparatus for production of anasphalt mixture, the apparatus having automatic controls for controllingthe ratio of asphalt to aggregate entering the mixing and drying zoneand for controlling the temperature of the asphalt coated aggregatedischarged; (5) to provide a process and apparatus for making asphaltpaving compositions at reduced capital cost and maintenance due to theuse of less processing and mixing equipment; and (6) to provide anapparatus for making asphalt paving compositions wherein the heated airflowing through the mixing drum and the material flow therethrough areparallel, enabling marked reduction in particulate emissions, andproduction of an even-temperature discharge asphalt mixture making itpossible to obtain more uniform compaction when the composition is laid.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of anasphalt plant employing the invention herein;

FIG. 2 is a vertical cross-sectional view of the mixing and drying drumof Fig. 4 illustrating parallel flow of the asphalt-aggregate mixtureand high velocity, heated gas through the mixing and drying zone;

FIG. 3 is a graph of therange of penetration values measured fromsamples taken at various points in the production operation;

FIG. 4 is an end view and partial side elevation of a modified drumdryer wherein a shield extends into the mixing and drying zone from theburner to avoid flame contact with the asphalt-aggregate mixture; and

FIG. 5 is a comparison of the No. 200 material which was introduced withthe aggregate into the mixing and drying zone with that retained in thedischarged asphaltaggregate mixture.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1, aggregateis dumped into a plurality of bins 10a, 10b, and of a compartmented coldfeed bin unit feeding to a positive weight control device 12 whichdelivers accurate weights of cold aggregate onto a conveyor belt 14running between the weight control means and chute 16 of a drum mixer. Aweight control device which may be used is a Merrick Feedoweight, ModelWLC, which has control gate screw setting and feed control settings.Accurate weights of feed according to the desired rate setting aredelivered by means of powered feed regulators which automatically adjustthe control feed gate without requiring the scale beam to do the workdirectly and without interfering with the sensitive response of theweighing mechanism of changes in load.

With such a control system, any variations of load due to changes inmaterial or density are instantly corrected by means of the automaticregulation of the feed The aggregate used need not be pre-conditioned,i.e., pre-dried or preheated. The aggregate should contain about 1-10%and preferably 35% water by weight based on the total weight of the mix.If the water content of the aggregate is not suflicient, as sensed by asuitable disposed moisture detector or by periodic random sample,additional water may be added. Water, if necessary, may be pumped bypump 20 to a spray bar 22 from a storage tank 18. Prior to the aggregateentering the drum mixer liquefied asphalt is metered onto the aggregateby a positive displacement pump 24 pumping the asphalt from a storagetank 26 through conduit 28 to a spray bar or nozzle 30 which sprays theasphalt across the Width of the aggregate on the conveyor belt 14. Theliquefied asphalt may be an untreated paving grade asphalt heated topumping temperature (generally 200 F. to 350 F an emulsified asphalt orcutback asphalt. The drum mixer comprises an inclined cylindrical drum32 supported for rotation about its axis on rollers (not shown), therollers journalled for rotation to frame 34. The drum mixer is rotatedby suitable power means (not shown). End covers 36 and 38 cover thecharge and discharge ends of the drum, the covers secured to the frame34. An opening in the charge end of the mixing drum receives chute 16,through which the asphalt-coated aggregate is directed. As illustratedby FIG. 1 the charge end cover 36 is several feet long with a gas or oilfired burner 40 axially disposed in the end thereof, the charge endcover lined on the interior with a refractory material. The burner 40 ismoved back of the infeed chute 16 a sufficient distance to preventdirect flame contact with the entering asphaltcoated aggregate. Fuel,either gas or oil, is supplied from storage tank 42 through line 44 ofthe burner. A blower 46 supplies air to the burner. Rather than move theinfeed chute ahead of the burner as illustrated by FIG. 1 the burner mayhave a tubular extension 41 or shield extending several feet into thedrum mixer, as illustrated by FIGS. 2 and 4. In this instance theasphalt-aggregate mixture is introduced through chute 16 locateddirectly above the burner. The chute is bifurcated to allow theasphalt-aggregate mixture to drop into the drum mixer on each side ofthe shield.

At the discharge end of the mixing drum is an exhaust opening and amaterial discharge chute 48. An exhaust fan 50, having a discharge stack52 connected to the exhaust opening, maintains gas flow through themixing drum at velocities ranging generally from *600 to 1500 feet perminute. The material flow and gas flow through the mixing drum areparallel. The interior of the drum mixer 32 is provided with flights 32ato mix the aggregate and asphalt as the mixture travels down the lengthof the dryer and to cascade the aggregate-asphalt mixture through theheated gas stream. FIG. 2 illustrates the cascading action of therotating drum mixer and the parallel flow of heated gases andasphalt-aggregate mixture. The fines 33 in the aggregate which create anair pollution problem are apparently initially coated with the asphaltmixture and, entrained in the gas stream, flow through cascadingcurtains of asphalt aggregate the entire length of the drum mixer andapparently adhere to the larger pieces of aggregate. By capturing thefines in the aggregate early in the process there is little discharge offines into the atmosphere. FIG. is a graphical comparison of aggregatehaving varying percentages of fines passing through a No. 200 sieve thatwere introduced into the drum mixer with that retained by the mixture ondischarge. The straight line indicates that the material introduced intothe drum mixer came out in the asphalt-aggregate mixture and was notlost in the mixing process or was not discharged into the atmosphere.Asphalt-coated aggregate is discharged from the drum mixer throughdischarge chute 48 onto belt conveyor 54 which conveys the heated mix toa storage bin 56 where it is loaded into trucks 58 for transfer to a uselocation.

A completely automated control system for burner control, such as onesold under the trade name Genco Genie Burner Control System, isemployed. Temperature sensing means are disposed to sense thetemperature of the mix discharge or stack emission temperature, thesensing means operatively connected to a controller controlling theburner 40. Separate feed controls for the cold aggregate, asphalt, andwater (if necessary) are also provided. The feed controls are preferablyinterlocked with one another. The feed control is of the type thatcontrols aggregate blend and meters aggregate to the mixing drum so thatthe gradation and volume metered into the mixing drum are maintainedessentially constant. The asphalt pump is a positive pump operativelyconnected to the feed control so that the gallons per minute of asphaltcomposition pumped into the mixing drum is maintained at a constant rateand at a predetermined weight ratio to the amount of aggregate meteredinto the mixing drum. Generally from 0.5% to 10% asphalt to aggregate ona weight basis is metered onto the aggregate. The asphaltaggregatemixture is discharged from the mixing drum at a temperature of around200 F. to 300 F. or even higher, and preferably at a temperature of 210F. t 220 F.

One of the major advantages of the process and apparatus of thisinvention is substantially complete elimination of air pollution causedby particulate emissions from the drum mixer without the need ofinstalling and maintaining auxiliary equipment. The combination ofparallel flow of the heated gas stream and an asphalt-aggregate mixturethrough the mixing and drying zone enables control of particulateemissions from the discharge stack. Fines in the aggregate must, ofnecessity, travel the length of the mixing and drying zone beforedischarge and, in so doing, travel through the cascading curtains ofasphalt-coated aggregate to which the fines adhere. Parallel flow of theheated gas stream and asphalt-aggregate mixture through the mixing anddrying zone also enables discharge of the mixture at a constanttemperature, thereby giving a product with improved compactioncharacteristics. Another advantage of parallel flow of the heated gasstream and asphalt-aggregate through the mixing and drying zone isreduced loss of the penetration value of the asphalt due to oxidationand hardening caused by direct contact of the asphalt composition withthe flame of the burner. FIG. 3 is a graph of penetration values forseveral days operation of (1) asphalt in its original form, (2) asphaltas recovered from the mixture during processing, (3) asphalt from corestaken several days after laying of a roadway surface and (4) asphaltsubjected to artificial aging and hardening by a standard testprocedure, the thin film oven test. The graph indicates that the asphaltretained 73% of its original penetration (from cores) as compared to 51%after the thin film oven test. Consistent penetration values of thecores was also noted. Sutficient water is associated with theasphalt-aggregate mixture when introduced into the charge end of themixing and drying zone adjacent the burner to protect the asphaltagainst adverse aging or hardening and loss of penetration value untilthe mixture is processed out of the flame area Within the mixing anddrying zone.

A drum dryer forty feet long and about ten feet in diameter powered forrotation by suitable power means is provided with a burner in the chargeend and an exhaust fan in the discharge end opposite the burner. Astainless steel shield about four feet in diameter extends around theflame area of the burner and about four feet into the in-' terior of thedryer. The drum dryer is provided with spiral flights extend from thecharge end to a point where the visible flame of the burner diminishes,generally four to five feet. These flights are designed to quicklyconvey the asphalt-aggregate out of the flame zone of the burner. Theflights following the spiral flights are segmented,

staggered flights designed to mix the asphalt-aggregate mixture andextend three or four feet beyond the spiral flights. The flightsfollowing the segmented rflights are straight flights as illustrated inFIG. 2 and extend to the discharge end of the drum dryer, these flightsdesigned to cascade the asphalt-aggregate through the heated gas streamcoursing through the drum mixer parallel to the movement of theasphalt-aggregate through the drum mixer.

Aggregate was conveyed through a chute located directly above the fireshield, as illustrated in FIG. 4. The aggregate was fed into the dryeron a thirty-six inch Wide belt at the rate of about 450 tons per hourwhen the plant was balanced for optimum operation. The aggregate was /8inch minus aggregate with about 65% passing a inch screen, 35% passingbetween /8 inch and A1 inch screen, and 10% Passing a No. 200 sieve. Theaggregate had a moisture content of about 7 weight percent. As theaggregate dropped into the chute, a 200 to 300 pentration paving gradeasphalt was sprayed onto the aggregate from a spray bar, the pavinggrade asphalt being previously heated to pumping temperature.

The asphalt-aggregate mixture entered the drum-dryer on both sides ofthe flame shield and was quickly conveyed by the spiral flights to apoint within the dryer where the visible flame from the dryersubstantially diminished, mixed by the second series of flights andcascaded through the heated gas stream to remove moisture therefrom anduniformly coat the aggregate with the asphalt. The asphalt-aggregate wasdischarged at a temperature of 220 F. to 225 F. n visual examination,the aggregate was totally coated with asphalt. The hot gases-coursing inparallel direction to the material movement through the drum-dryertravelled through an expansion chamber, then into a four foot diameterpipe, and were discharged to the atmoshpere at a temperature of 450 F.to 550 F. The exhaust fan pulling the air through the drum-dryeroperated at about 80,000 cubic feet per minute and was powered by a 250horsepower electric motor. The asphalt discharged had a moisture contentof about 1% by weight and an asphalt content of about 6.5% by Weight.

After the plant was balanced for optimum operation, there was no visualevidence of any dust emanating from the stack-only heat Waves plus aslight tinge of blue air which dissipated within 100 yards of the stack.

The embodiments of the invention in which a particular property orprivilege is claimed are defined as follows:

1. A continuous process for making asphalt paving compositions bycoating of aggregate with a liquefied asphalt composition consisting ofpaving-grade asphalt or a cut-back asphalt wherein the process resultsin reduced discharge of aggregate fines into the atmosphere, the pavingcompositions having uniform compaction due to even temperature of theasphalt-coated aggregate on discharge, comprising:

providing a confined travel path between a charge station and adischarge station wherein the liquefied asphalt composition-aggregatemixture is mixed to uniformly coat the aggregate with the liquefiedasphalt composition,

moving a high velocity, heated gas stream along the confined travel pathfrom the charge station to the discharge station,

conveying an unconditioned aggregate having about 1% to 10%water basedon the weight of the aggregate associated therewith to the chargestation, contacting the aggregate with the liquefied asphalt compositionprior to any substantial heating of the aggregate by the heated gasstream along the confined travel path, the liquefied asphalt composition6 initially adhering to and capturing the fines in the aggregate, --.and1 tumbling and cascading the aggregate-liquefied asphalt compositionmixture through the heated gas stream along substantially the totallength of the confined travel path of the mixture to complete coating ofthe aggregate with the liquefied asphalt composition and to removemoisture from the mixture without substantial hardening of the asphaltcomposition, the coated fines adhering to the larger asphalt-coatedaggregate and not becoming entrained in the heated gas stream anddischarged as an air pollutant.

2. The method of claim 1 wherein 0.5% to 10% of the liquefied asphaltcomposition based upon the total weight of the asphalt-aggregate mixtureis applied to the aggregate.

3. The method of claim 2 wherein the liquefied asphalt has a temperatureranging from about ambient to 350 F. and the heated gas stream is causedto flow along the confined path at a rate of from 600 to 1500 feet perminute.

4. A continuous method for making asphalt paving compositions by coatingof aggregate with a liquefied asphalt composition consisting ofpaving-grade asphalt heated to pumping temperature or a cut-back asphaltwherein the process results in less aging or hardening of the asphaltcomposition and reduced amounts of air pollutants resulting fromdischarge of fines associated with the aggregate into the atmosphere,the paving compositions having uniform compaction due to eventemperature of the asphalt-coated aggregate on discharge, comprising:

providing an inclined drum mixer having a confined travel path betweenan upper charge end and a lower discharge end, moving a heated gasstream along the confined travel path in the drum mixer from the uppercharge end to the lower discharge end at a rate of from 600 to 1500 feetper minute,

conveying an unconditioned aggregate at substantially ambienttemperature having about 1% to 10% water based on the weight of theaggregate associated therewith to the charge end of the drum mixer,

contacting the conveyed aggregate with the liquefied asphalt compositionsubstantially simultaneously on introduction of the conveyed aggregateinto the charge end of the drum mixer, the liquefied asphalt compositioninitially capturing the fines in the aggregate, and

tumbling and cascading the aggregate-liquefied asphalt mixture throughthe heated gas stream along substantially the total length of theconfined travel path of the mixture to uniformly coat the aggregate withthe liquefied asphalt composition and to remove moisture from themixture without substantial hardening of the asphalt composition, thefines associated with the aggregate adhering to larger asphalt-coatedaggregate and not becoming entrained in the heated gas stream fordischarge as an air pollutant.

5. The method of claim 4 wherein the gas stream is heated by combustingan air-fuel mixture at the charge end of the drum mixer, wherein the hotcombustion gases are caused to move at a realtively high velocity alongthe confined path and wherein the aggregate and liquefied asphalt areintroduced into the charge end of the drum mixer to avoid direct contactwith the flame of the combusted fuel-air mixture.

6. A continuous method for making asphalt paving compositions by coatingof aggregate with a liquefied asphalt composition consisting ofpaving-grade asphalt heated to pumping temperature or a cutback asphaltwherein the process results in less aging or hardening of the asphaltcomposition and reduced amounts of air pollutants due to discharge offines associated with the aggregate into the atmosphere, the pavingcompositions having uniform 7 8 compaction due to even temperature ofthe ,asphaltsta ntially, the total length of the, confinedtravel pathcoated aggregate on discharge, comprising: a V p v q of the mixture touniformly coat the aggregate with providing an inclined drum mixerhaving a confined the liquefied.asphalt-composition, the.finesassociated travel path hetwcenan upper charge end and a-lower vWiththe aggregate adhering to largerfasphaltwcoated discharge end, aggregateand not becoming entrained in the heated combusting an air-fuel mixtureat the charge end of the gas stream for dischargeas an air pollutantdrum mixer, moving the hot combustion gases through the con- ReferencesCited fined travel path in the drum mixer from the upper UNITED STATESPATENTS charge end to the lower discharge end at a rate of v I i from600 to 1500 feet per minute, 1,240,481 9/1917 Popkess 259 158 conveyingan unconditioned aggregate at substantially 2,028,745 1/1936 Hendrick259-157 ambient temperature having about 1% to 10% Water 2,188,7981/1940 Smith 259-155 based on the weight of the aggregate associated3,423,222 1/1969 McConnaughay 106-278 therewith to the charge end of thedrum mixer, 3,614,071 10/ 1971 Brock 2593 contacting the conveyedaggregate with the liquefied 3,693,945 9/ 1972 Brock 259158 asphaltcomposition substantially simultaneously on introduction of the conveyedaggregate into the JOSEPH SCHOFER Pumary Exammer. charge end of the drummixer, the liquefied asphalt H. I. LILLING, Assistant Examinercomposition initially capturing the fines in the ag- Us.

gregate, and tumbling and cascading the aggregate-liquefied asphalt 106273 259 158 mixture through the heated gas stream along sub-

